I. Field
The present invention relates generally to communication, and more specifically to techniques for transmitting data by a wireless device.
II. Background
In a wireless communication system, a radio frequency (RF) signal transmitted by a wireless device (e.g., a cellular phone) may reach a base station via multiple signal paths. These signal paths may include a line-of-sight path and reflected paths, which are created by reflections of radio waves in the environment. The base station may thus receive multiple instances of the transmitted RF signal. Each received signal instance is obtained via a different signal path and has a complex gain and a propagation delay determined by that signal path. The received signal instances may add constructively at the base station and produce a received signal having a larger magnitude. Conversely, the received signal instances may add destructively and produce a received signal having a smaller magnitude. Different received signal levels may thus be obtained depending on reinforcement or cancellation of the received signal instances. Reinforcement is usually not problematic. However, cancellation may cause the received signal level to drop by a large amount, e.g., by up to 40 decibels (dB). The received signal is said to be in a “fade” when attenuated by a large amount due to cancellation.
Some wireless communication systems, such as Code Division Multiple Access (CDMA) systems, utilize power control to mitigate the deleterious effects of fading. With power control, the transmit power of the wireless device is adjusted either up or down as needed to achieve a target signal-to-total-noise ratio (SNR) at the base station. If the base station detects that the received SNR for the wireless device drops below the target SNR, e.g., due to changes in the wireless environment, then the base station sends transmit power control (TPC) commands to direct the wireless device to increase its transmit power. The wireless device may vary its transmit power over a wide range in order to maintain the received SNR at or near the target SNR. For example, if a fade causes the received signal at the base station to drop by 20 dB, then the wireless device would be instructed to increase its transmit power by approximately 20 dB (or 100 times higher) in order to maintain the desired SNR at the base station.
Many wireless devices are portable and powered by internal batteries. The use of high transmit power to combat fading depletes battery power, which shortens talk time. There is therefore a need in the art for techniques to reduce transmit power and extend talk time for such a portable wireless device.